When the user keeps on adding mail rules, at one point of time, it exceeds the fixed maximum allowed size and user can no longer save that mail rule. So the normal user has no idea about the mail rule size is exceeding in total (32K) and therefore new rules cannot be saved. To avoid this confusion, always show how much size is remaining like Quota for creating mail rules. This will help user and Administrator to understand and manage the mail rules in better way.
The display can be as simple as ("26K used out of 32K").